Conventionally, as conductive fibers, there are known to be fibers that have a fiber surface coated with metal such as copper, or that are interwoven with fine wire of carbon or metal, conductive fibers that is a conductive polymer formed into a cord-like shape, and so on. These conductive fibers are widely used in biological electrodes, biointerfaces, antistatic clothing, and the like. However, conductive materials such as metal or carbon are hydrophobic and hard, resulting in the problem that they are poorly compatible with applications involving contact with body surfaces and body tissues of living organisms that are highly moist and soft. For example, in the case where a biological electrode is set on a body surface, adhesion and direct conduction with the body surface are inhibited when the biological electrode is composed of hard hydrophobic material. Consequently, it is necessary to separately prepare and use conductive paste (jelly) that electrically connects the bioelectrode and the body surface.
In recent years, as material having satisfactory compatibility with living bodies, development has advanced with respect to conductive fibers that are molded into thread-like form by extruding an aqueous solution of PEDOT-PSS {poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid)} that is a conductive polymer with particularly good conductivity and hydrophilicity from a nozzle into an acetone coagulation bath. Commercialization thereof is also under study (see, e.g., Non-Patent Document 1).
However, when conductive fibers composed of the aforementioned PEDOT-PSS are used in a high-humidity environment, there is the problem that the PEDOT-PSS absorbs moisture, and that strength (particularly tensile strength) declines. Moreover, conductive fibers composed of PEDOT-PSS expand when they absorb moisture, and conversely contract when dried. Consequently, as a result of cracking that occurs within the fibers, or fracturing of the fibers, there is the problem that the conductivity of the fibers tends to decline or be lost.
Clothing is susceptible to becoming water-soaked during use due to rain or perspiration. The usage environment of biological electrodes and biointerfaces is essentially a high-humidity one. Accordingly, resolution of the aforementioned problems is required in order to utilize PEDOT-PSS with its excellent conductivity and hydrophilicity in these wide-ranging applications.
In addition to the aforementioned problem that conductive fibers composed of PEDOT-PSS undergo a marked decline in strength when they contain moisture, there is also the following problem. That is, the aforementioned fibers produced by the wet-spinning method recorded in the Non-Patent Document 1 are fine fibers with a diameter of approximately 10 microns. Consequently, there is the problem that handling is difficult, and that strength is insufficient when dry. Furthermore, the aforementioned fibers are highly rigid, and have a coarse feel. Consequently, there is also the issue that they are deficient in imparting the suppleness required by applications such as clothing.
On the other hand, biological electrodes of the body surface attachment type are widely used for recording bioelectrical signals such as brain waves, event-related potential, evoked potential, electromyograms, and electrocardiograms, and for electrically stimulating living bodies. (Biological electrodes of the body surface attachment type may be hereinafter referred to simply as “biological electrodes.”)
The biological electrodes that have heretofore been in wide use are composed of a metallic electrode plate and gel or paste containing an electrolyte solution. The basic structure of these biological electrodes uses (applies) gel or paste between the metallic electrode plate and the skin surface to fix the electrode plate to the skin surface. Due to attachment of the biological electrode, a prescribed position on the skin surface is constantly sealed. Consequently, particularly when there is continuous use over a long period, not only may a sense of discomfort or itchiness arise due to moldering perspiration, but also contact dermatitis or bacterial infections or the like may occur. Such problems of the prior art require resolution.
In the various countries with a growing elderly population, there are an increasing number of cases where monitoring of biosignals such as electrocardiograms is conducted over long periods. As various skin functions decline in elderly people, highly adhesive stick-on electrodes using conventional adhesive tape or the like tend to produce dermatitis or uncomfortable sensations such as itchiness. Furthermore, the trouble frequently occurs that wearers who exhibit dementia or nighttime delirium themselves remove the biological electrode, and such problems require corrective measures.
With respect to conventional biological electrodes that are susceptible to the aforementioned problems, gel or paste containing an electrolyte solution is used between the skin and the metallic electrode plate. In the case where a biological electrode is set on a skin surface via gel or paste, it is necessary to increase the contact area of the electrode. The reason is that, due to the low conductivity of gel or paste, it is necessary to reduce electrode resistance by expanding the area of contact with skin. However, expansion of the electrode contact area is the primary cause of occurrence of the aforementioned problems.
Thus, the existing biological electrode configuration that relies on electrolytic gel or paste is deficient in wear comfort, and inhibits further downsizing and increased density of the electrode.
On the other hand, an implantable biological electrode is necessary in order to have an external device accurately and efficiently receive electrical signals from within a living body, and conversely to have an external device transmit electrical signals into a living body. Signals of action potential and synaptic potential of nerve cells are particularly weak. Consequently, if an electrode is not set very close to the cells, there are many signals that would be difficult to measure and input. Even apart from the nervous system, implantable biological electrodes are widely used in cardiac pacemakers, cochlear implants, and so on. Moreover, with respect to future human interfaces, development is advancing with respect to implantable biological electrodes for brain-machine interface and the like.
The living body consists of an abundance of water and electrolytes, as well as soft tissue. In contrast, conventional biological electrodes for implantation into living bodies are fabricated using conductive material that is hard and hydrophobic such as metal or carbon. Consequently, there have been problems of mechanical and electrochemical compatibility between conventional biological electrodes and biological tissue.
In particular, there are the problems that inflammation occurs due to mechanical stress arising at the boundary of the biological electrode and the biological tissue, and that tissue is impaired (invaded).
Within biological tissue, as well, the following problems may occur when electrodes are implanted into nerve tissue of central nervous system. That is, inflammation provoked by minute injuries to nerve tissue may gradually expand, resulting in degeneration and separation of nerve cells around the electrode, and inhibiting measurement and stimulation (signal input). Particularly with respect to permanent implantation of electrodes into tissue, there is loss and glial scarring of nerve cells, leading to reduced efficiency of electrical stimulation, and degradation and loss of measured waveforms. In addition, there is also the risk of causing functional impairment to nerves due to the loss of nerve cells. In view of these matters, corrective measures are required.